Many women (and men) find that the spark goes out of sex in long-term relationships. One of the experts in this area is Esther Perel, who has written some very helpful books – I often recommend Mating in Captivity.
She was recently interviewed on the BBC, and I thought the article Seven Things We’re Getting Wrong About Sex in Long-Term Relationships had some excellent tips. (You can also listen to the full program at the link.)
(We don’t get any kickbacks from these recommendations.)